1. Field of the Invention
The present invention relates to visible and near ultraviolet wavelength lasers, and more specifically to diode-pumped up-conversion alkali lasers (DPALs).
2. Description of Related Art
With the coming of the Internet and the explosive growth in data communications it enabled, there has been a concomitant growth in the demand for ever-more capable visual displays in the form of electronic cinema, home theater, desktop, and mobile displays. The growth in data generation and communications has also created an accelerating demand for high density data storage, taking many physical forms including optical data storage in video DVD disks, and in optically written holograms in polymer coated disks. Advanced realizations of high-performance displays and data storage call for the use of compact, efficient, and low-cost visible and near ultraviolet laser sources. Direct view displays based on lasers require laser sources emitting in the red, green, and blue spectral regions. Optical data storage recording media achieve high recording density by making small spots in the recording medium, whose spot diameter depends inversely in the square of the wavelength of the laser-marking source. Thus optical data storage devices benefit from compact, efficient shorter wavelength laser sources. DVD disks originally utilized semiconductor laser diodes operating at 780 nm, progressed in recent times to a wavelength of 650 nm, and call for laser sources that will emit at a wavelength near xcx9c400 nm in the near future. Thus there are continuing, and growing needs for compact, efficient, and cost-effective visible and near ultraviolet laser sources.
More specifically, there is a large and growing demand for commercial projection displays [1] with ever-higher technical performance characteristics (higher resolution, higher brightness, larger screen size, more saturated color gamut, higher efficiency, and lower cost, etc.). The xenon arc lamp found in most projection displays is technologically the weakest link in achieving displays with the desired characteristics. The arc lamp is generally limited in brightness because its output light is radiated into all spatial directions, is inefficient in producing useful visible light, produces a great amount of waste heat, and has an awkwardly short lifetime usually measured in 100""s of hours.
Visible red, green, and blue (RGB) laser sources offer the prospect for overcoming most of the shortcomings of incoherent arc lamp sources. Lasers are comparatively much brighter than lamps, emit relatively pure colors that enable very high gamut saturation, and can be scaled in output power sufficient to project bright high-resolution images on very large screens. The most developed visible laser sources for projection displays are based on diode-pumped solid state lasers, or DPSSLs, (such as Nd:YVO4), The near infrared radiation from the DPSSL is frequency-doubled in a nonlinear crystal, producing either red (xcx9c640 nm), green (xcx9c532 nm) or blue (473 nm) visible light [2]. These laser sources have working efficiencies of 2-5 percent, produce output powers up to the watt range, but have proven to be many times too expensive for wide-spread use in consumer display applications.
Lower power (10-100 mW) laser-based visible sources are being developed, based on direct frequency doubling of the near-infrared radiation from a stripe laser diode in a nonlinear crystal [3]. At these lower powers it is necessary to use a guided wave structure fabricated in the nonlinear harmonic doubler crystal, so that a sufficient interaction length is provided for significant harmonic generation. The most promising results regarding output power and conversion efficiency have been obtained using quasi-phase-matched periodically-poled nonlinear materials such a lithium niobate and lithium tantalate [3]. In these devices, the near diffraction-limited radiation from the stripe laser diode is focused into a channel waveguide (a few microns in width) that is fabricated in a planar wafer made of the nonlinear converter crystal. In order to achieve reasonable conversion efficiencies (10-20% or so) the fundamental wave in the waveguide must have an intensity of at least a few hundred kW/cm2. Such an intensity is high enough that light-induced photorefraction occurs. This phenomenon spoils the phase-matching condition for efficient harmonic generation and greatly limits the operating lifetime of the device, especially at the higher output levels [4]. This problem has proven to be most difficult in generating shorter visible wavelengths (e.g., s blue). Also the precision required to fabricate micron scale diode stripe lasers and couple them efficiently into narrow width single-mode waveguides is a challenging and relatively expensive task to perform.
Thus, the market demand for relatively lower-cost, compact, efficient, high-power (0.1 to 10 watts), and long-lived ( greater than  greater than 20,000 hours) visible (especially blue) laser sources continues unfulfilled. The present invention is offered as a solution to this market need.
In addition to high performance displays, consumer demand has continued to grow in the past decade [5] for video DVD disks with ever-higher recording densities. Commercial video DVD disks containing a full 2 hour-long feature film have been realized with the development of red (xcx9c650 nm) laser diodes. Future higher density DVD (or DVR) disks [6] are awaiting the development of a compact, efficient laser source emitting at a shorter wavelength (xcx9c420-400 nm). Laser diodes produced from the AlGaInN compound semiconductor material system are in early development for this application. AlGaN laser diodes emitting several tens of milliwatts at a wavelength of xcx9c410 nm have been demonstrated [7] and are in early commercial evaluation. While technically adequate, the current manufacturing methods of such diodes is a low-yield process, owing to the lack of a suitably lattice-matched substrate upon which to epitaxially grow these laser diode devices [8]. Thus, novel compact, efficient, low-cost laser sources in the 420-400 nm spectral region continue to be of commercial interest.
In addition to video DVD disk recorders and players, yet higher data density and access rates are needed to implement massive data storage devices for data rich computer network applications. Holographic data storage techniques have been under intense development in the past decade, and new polymer recording media have been developed for commercial and consumer products [9]. Holographic data storage devices will require practical, short wavelength (xcx9c400-410 nm) lasers emitting several tens to up to a xcx9c100 milliwatts of laser power. Such laser sources are also useful as a compact fluorescence excitation source for various biomedical research and diagnostic applications (such as cancer detection, DNA sequencing, and reading proteomic assays, etc.).
In light of the foregoing, needs continue for the invention and development of efficient, compact, long-lived, visible laser sources operating in the xcx9c400-470 nm spectral range. The present invention addresses those needs.
It is an object of the present invention to provide an up-conversion diode-pumped alkali laser (UC-DPAL.
It is another object of the invention to provide to provide a laser cavity formed by an input mirror and an output mirror, resonant at a wavelength xcex03 or xcex04 corresponding to wavelengths of the D1xe2x80x2 or D2xe2x80x2 transitions of an alkali atomic vapor.
Another object of the invention is to provide a gain medium within a resonant cavity, where the gain medium comprises a mixture of one or more buffer gases and an alkali vapor whose D1xe2x80x2 or D2xe2x80x2 transition wavelengths match that of the resonant laser cavity.
Still another object of the invention is to provide a semiconductor diode pump laser (or laser array) emitting at a wavelength suitable for optically exciting a laser gain mixture of one or more buffer gases and an alkali vapor.
Another object of the invention is to provide a semiconductor diode pump laser (or laser diode array) emitting at a wavelength suitable for further optically exciting alkali atoms excited by a first pump laser, to the n 2D3/2 (or similar) electronic level of the alkali atom.
Another object of the invention is to provide a method for converting the substantially-divergent, multi-spatial-mode radiation of semiconductor diode laser pump arrays into a near diffraction-limited, near-single-spatial-mode, coherent laser radiation at a wavelength shorter than those of either pump.
These and other objects will be apparent to those skilled in the art based on the disclosure herein.
The use of an alkali atomic vapor element as laser active specie in a near infrared Diode-Pumped Alkali Laser (DPAL) has been disclosed [10] in U.S. patent application Ser. No. 10/000,508, titled xe2x80x9cDiode-Pumped Alkali Laserxe2x80x9d filed Oct. 23, 2001, and incorporated herein by reference. In the basic DPAL device, excitation to the n 2P3/2 electronic level by a single diode laser pump source leads to a population inversion between the first excited electronic 2P1/2 level and the ground 2S1/2 level, permitting the construction of efficient, high-power, compact DPAL laser oscillators in the near infrared spectral region. The present invention extends the single-step excitation DPAL to a two-step excitation, or up-conversion DPAL to produce efficient, powerful laser operation in the visible blue and near UV spectral regions (viz., in the range 460-323 nm). The present invention describes an apparatus and method that efficiently sums the energy of two, near-infrared diode pump photons in alkali vapor atoms, followed by stimulated emission to their electronic ground levels.
In the basic infrared DPAL, only the ground and first two excited energy levels are involved in laser action. In the UC-DPAL device, additional higher lying electronic levels are involved in generating visible laser emission. In the UC-DPAL device, two diode pump sources are utilized. The first pump, P1, is set to the wavelength of either of the first resonance (so-called) D1 or D2 transition wavelengths (D1: n 2S1/2-n 2P1/2, or D2: n 2S1/2-n2P3/2). The second pump, P2, is set to a wavelength that equals the wavelength of a transition between either the n 2P1/2 level or the n 2P3/2 level, and the n 2D3/2 level (or possibly another 2DJ level, not shown). With both pump excitation sources present, alkali atoms are successively excited from the ground n 2S1/2 electronic level, into either the n 2P1/2 or n 2P3/2 levels, and subsequently into the n 2D3/2 level. In the presence of an appropriate buffer gas mixture, the alkali atom populations excited to the n 2P1/2 and n 2P3/2 levels come into thermal equilibrium with each other, characterized by a temperature equal to that of the buffer gas, due to rapid collisional mixing (exchange of energy) between these levels by the buffer gas. Similarly, due to collisional mixing among the n 2D3/2, n+1 2P1/2 and n+1 2P3/2 levels due to presence of an appropriate buffer gas, the alkali atom population excited by the second step pump rapidly comes to thermal equilibrium with the latter two levels, characterized by the temperature of the buffer gas. With the appropriate excitation fluxes from the first and second diode pump sources, a population inversion is generated between the n+1 2P1/2 and n+1 2P3/2 levels and the ground n 2S1/2 level. When the doubly-excited alkali/buffer-gas mixture is contained with an appropriate laser cavity, laser action is generated in either of the two xe2x80x9csecond series Dxe2x80x2-transitionsxe2x80x9d of the alkali atoms: D1xe2x80x2: n+1 2P1/2xe2x88x92n 2S1/2; D2xe2x80x2: n+1 2P3/2xe2x88x92n 2S1/2.